The present invention relates to phase locked loop circuits, and more particularly to such circuits with a wide lock-in range.
In the "Triax" television camera system, a single double shielded cable (triaxial cable) is used for carrying signals from the camera head (CH) to the central processing unit (CPU) and vice versa. Among several other signals, three wide band video signals containing red, green and blue information of a scene being viewed by the CH have to be transmitted to the CPU. In a particular system the method chosen to do this is frequency multiplexing, with three different crystal controlled carriers on 16.5, 28.5 and 40.5 MHz respectively. These carrier frequencies can also be chosen differently. Double sideband suppressed-carrier amplitude modulation is used.
In order to demodulate such a signal at the CPU, the carrier has to be regenerated with a phase locked loop (PLL) system having a demodulator, integrator and an inductor-capacitor (LC) voltage controlled oscillator (VCO). As a phase reference for the PLL, the video signal during certain of its intervals, such as the horizontal and vertical blanking periods, can be used. In fact, both blanking periods are used in the disclosed embodiments to generate a phase reference by sampling an error signal from the demodulator, which error signal is a sine wave when the PLL is not locked onto the carrier frequency. However, such a sampled system can sample nearly equal positive and negative portions of the sine wave when the frequency of the sine wave corresponds to the horizontal sampling frequency or a harmonic thereof. The result is little or no error voltage applied to the integrator in the PLL. Hence a frequency control voltage for the VCO derived from the integrator remains unchanged, and therefore the PLL may lock onto a sideband of the suppressed carrier, which sideband has a frequency that differs from the carrier frequency by the sampling frequency or a harmonic thereof.
Further, a wide frequency pull-in range for the PLL is necessary because of the tolerances and frequency drift of the LC VCO. A more stable crystal controlled oscillator is not possible, because at the frequencies involved, the crystals have to work at their third harmonic and can only be pulled over a very limited frequency range (maximum a few hundred hertz), which is not enough to deal with tolerance and temperature drift of the crystal oscillator in the camera head. A wide PLL pull-in range requires a small time constant in the integrator; however, a large time constant is desired for increased noise immunity.
It is therefore desirable to have a PLL that has a wide pull-in range, high noise immunity, and which avoids false lock ups.